This invention pertains generally to process controllers and more particularly to a digital system and method for generating time variant electrical analog signals for controlling processes, machines and other equipment.
Time variant electric analog signals are utilized in a wide variety of industrial and other applications for controlling a process or the operation of a machine, instrument or other equipment to provide accurate results which are readily repeatable. Such applications include the start-up of furnaces and reactors, the introduction of reactants, control of welding currents, regulation of flow and control of pressure and temperature. The desired control signals are often derived indirectly and have complex wave forms.
Heretofore, process control signals have been generated primarily by mechanical techniques or a combination of electrical and mechanical techniques. In one widely used prior art approach, a cam is formed with a profile corresponding to the wave form of a desired control signal, and a cam follower driven by the cam controls the movement of a potentiometer to generate the analog signal. In another prior art approach, a line corresponding to a desired wave form is scribed into an electrically conductive layer on an insulative card, and a servomotor controlled pick-up device follows the line and controls the operation of a potentiometer to generate the desired signal. Although widely used, such systems have certain limitations and disadvantages. For example, mechanical inertia prevents the generation of signals whih change instantaneously in amplitude and it is generally not possible for the control signals to begin and end at different levels. The mechanical parts are subject to friction and wear which can lead to inaccurate results and ultimate failure of the system, and the range of time bases which such systems can provide is limited.